In an optical transmission system, when signals are transmitted at higher rates on the order of several tens of Gbps, quality deterioration due to polarization mode dispersion becomes non-negligible. As the influence of polarization mode dispersion increases, signals cannot be normally transmitted/received in some cases. Approaches that are generally used to reduce the influence of polarization mode dispersion include devising a transmission signal modulation scheme, utilizing a transmission channel which is less affected by polarization mode dispersion, and compensating for waveform for distortion.
One method of devising a transmission signal modulation scheme is to use multi-value modulation. A transmission bit rate can be represented by the product of the number of codes per bit and a transmission rate. Generally, the number of codes per bit is one in optical transmissions, but the same bit rate can be achieved at a lower transmission rate by increasing the number of codes.
Since the influence of polarization mode dispersion increases in proportion to the transmission rate, the influence of the polarization mode dispersion can be reduced by increasing the number of codes per bit, i.e., using a multi-value modulation. However, in most cases, the realization of a multi-value optical modulation such as QPSK (Quadrature Phase Shift Keying) is accompanied by a high cost, an increase in the size of device, and technical difficulties. Therefore, utilization of optical multi-value modulation gives rise to problems in terms of the cost, size, and the degree of technical difficulty.
While deterioration in the quality of transmission signals caused by the polarization mode dispersion is mitigated by utilizing a transmission channel which is less affected by polarization mode dispersion, replacement of a transmission channel involves a large scale construction work, and may lead to a situation where the provision of services can not but be interrupted in some cases. Thus, problems arise in terms of cost and availability.
There is also a method of preventing a deterioration of the quality of transmission signals by correcting waveform distortion to equalize a waveform distorted by polarization mode dispersion. This method depends on which information is used as a basis to control an equalizer. At present, no method is available for correctly monitoring the amount of polarization mode dispersion, including up to higher order components, in service, so that direct control cannot be conducted on the basis of the amount of polarization mode dispersion. Since a monotonously increasing correlation exists between the magnitude of polarization mode dispersion and the bit error rate (BER) of a received signal, indirect control can be conducted on the basis of BER.
However, since factors causing a BER deterioration are not limited to polarization mode dispersion, it is necessary to extract the deterioration caused by the polarization mode dispersion from a BER deterioration amount, but such a method is not available at present. In other words, correct polarization mode dispersion equalization control cannot be conducted with the use of BER information. Accordingly, a waveform equalization scheme implies technical difficulties at present.
Also, any of the conventional techniques described above is limited in the amount of improvements in polarization mode dispersion immunity, and the amount of improvements is not often proportionate to the cost. A preferable method for providing improvement is one in which improvement, though is not necessarily the highest degree of improvement, can be obtained at a cost that is only half as expensive as a method which obtained the highest amount of improvements attainable using state of the art techniques.
Referring now to Patent Document 1, a technique is disclosed for branching a transmitted optical signal into two lines and selecting a signal on a line which shows smaller polarization mode dispersion, in order to mitigate the influence of polarization mode dispersion which occurs within an optical fiber.    Patent Document 1: JP-6-334606